April 2005

1

M9999-042205

MIC2172/3172

Micrel, Inc.

slave’s. The master MIC2172’s oscillator frequency is in-

creased up to 135kHz by connecting a resistor from SYNC to

ground (see applications information).

The MIC2172/3172 is available in an 8-pin plastic DIP or

SOIC for –40°C to +85°C operation.

Features

•1.25A, 65V internal switch rating

• 3V to 40V input voltage range

• Current-mode operation

•Internal cycle-by-cycle current limit

• Thermal shutdown

• Low external parts count

• Operates in most switching topologies

• 7mA quiescent current (operating)

•<1µA quiescent current, shutdown mode (MIC3172)

• TTL shutdown compatibility (MIC3172)

• External frequency synchronization (MIC2172)

• External frequency trim (MIC2172)

•Fits most LT1172 sockets (see applications info)

Applications

• Laptop/palmtop computers

• Toys

• Hand-held instruments

•Off-line converter up to 50W

(requires external power switch)

•Predriver for higher power capability

• Master/slave configurations (MIC2172)

General Description

The MIC2172 and MIC3172 are complete 100kHz SMPS

current-mode controllers with internal 65V 1.25A power

switches. The MIC2172 features external frequency syn-

chronization or frequency adjustment, while the MIC3172

features an enable/shutdown control input.

Although primarily intended for voltage step-up applications,

the floating switch architecture of the MIC2172/3172 makes

it practical for step-down, inverting, and Cuk configurations

as well as isolated topologies.

Operating from 3V to 40V, the MIC2172/3172 draws only

7mA of quiescent current making it attractive for battery

operated supplies.

The MIC3172 is for applications that require on/off control of

the regulator.

The MIC3172 is externally shutdown by

applying a TTL low signal to EN (enable). When disabled, the

MIC3172 draws only leakage current (typically less than

1µA). EN must be high for normal operation. For applications

not requiring control, EN must be tied to V

The MIC2172 is for applications requiring two or more SMPS

regulators that operate from the same input supply. The

MIC2172 features a SYNC input which allows locking of its

internal oscillator to an external reference. This makes it

possible to avoid the audible beat frequencies that result from

the unequal oscillator frequencies of independent SMPS

regulators.

A reference signal can be supplied by one MIC2172 desig-

nated as a master. To insure locking of the slave’s oscillators,

the reference oscillator frequency must be higher than the

MIC2172/3172

100kHz 1.25A Switching Regulators

Typical Applications

C4

470µF

D2

1N5818

C1

22µF

C2

1µF

R3

1k

R4*

5V, 0.25A

* Optional voltage clipper (may be req’d if T1 leakage inductance too high)

C3*

D1*

R1

3.74k

1%

R2

1.24k

1%

4V to 6V

T1

Enable

Shutdown

1:1.25

MIC3172

FB

EN

GND

P1 P2

S

COMP

Figure 2.

MIC3172 5V Flyback Converter

+5V

(4.75V min.)

D1

1N5822

MIC2172

FB

C1*

22µF

R3

1k

C3

1µF

R1

10.7k

1%

C2

470µF

+12V, 0.14A

L1

27µH

* Locate near MIC2172 when supply leads > 2"

R2

1.24k

1%

SYNC

GND

P1 P2

S

N/C

COMP

Figure 1.

MIC2172 5V to 12V Boost Converter

Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com